In ITS (Intelligent Transport System), vehicle to vehicle communication for exchanging information between vehicles by use of radio transmission has been investigated. In vehicle to vehicle communication, onboard apparatuses (which will be referred to hereinbelow as onboard units) each are installed in plural vehicles, and each performs communication with other onboard units residing within the communication area of the own unit.
Further, in ITS, multi-hop communication that enables data forwarding by way of multiple onboard units is also attempted. In multi-hop communication, in order for a source onboard unit to establish communication with a destination onboard unit residing beyond the communication area of the own onboard unit, onboard units that are present between the source onboard unit and the destination onboard unit successively forward communication data addressed to the destination onboard unit.
It has been also investigated that when the road on which a vehicle that has an installed onboard unit is travelling is congested, the onboard unit transmits traffic jam information on the current status of that traffic congestion, and the traffic jam information is notified to onboard units residing out of the congested section of the road. In this case, as a communication area communicable with onboard units, a roadside unit having a wider communication area than that of onboard units is used. Upon receiving the traffic jam information, the roadside unit transmits the traffic jam information all at once to all the onboard units residing within the communication area.
In Patent Document 1 (JP2008-205817A), an onboard unit that performs communication with a roadside unit and neighboring onboard units is disclosed. The onboard unit disclosed in Patent Document 1 is able to perform communication with a roadside unit by way of neighboring onboard units when the vehicle which has an installed onboard unit runs outside the communication area of the roadside unit. Accordingly, when the onboard unit described in Patent Document 1 transmits information which is addressed to the roadside unit in a situation where a vehicle which the onboard unit has been installed is travelling outside the communication area of the road side unit, the onboard unit transmits information to neighboring onboard units. Then, when a neighboring onboard unit that is communicating with the roadside unit (which will be referred to hereinbelow as a “communicating onboard unit”) receives the information addressed to the roadside unit, the communicating onboard unit forwards the information addressed to the roadside unit, to the roadside unit.